Among the batteries which are in a practical use, a lithium ion secondary battery shows the highest energy density, and has been widely used for the compact electronics. Also, in addition to the purpose of making compact, it is also hoped to be used for automobiles as well. Under such situation, the lithium ion secondary battery are demanded to have a longer lifetime, and further improved safety property.
The lithium ion secondary battery comprises; positive electrodes and negative electrodes including the electrode composite layer generally supported between a current collector, a separator, and nonaqueous electrolyte solution. The electrode composite layer includes electrode active materials having the average particle diameter of 5 to 50 μm or so and a binder. The positive and negative electrodes are produced by forming the electrode composite layer by coating composite slurry including the electrode active material powder onto the current collector. Also, as the separator for separating the positive electrode and the negative electrode, an extremely thin resin film having the thickness of 10 to 50 μm or so is used.
Usually, the lithium ion secondary battery is produced by going through the step of stacking the electrodes and the separators, and the cutting step of cutting into a predetermined electrode shape or so. However, while going through this series of production steps, the electrode active materials falls off from the electrode composite layer, and the portion of the fallen electrode active material may be included in the battery as a contaminant.
Such contaminant has the particle diameter of 5 to 50 μm or so. Since said particle diameter is about the same as the thickness of the separator, it can penetrate through the separator in the assembled battery, thereby may cause the short circuit. Also, when the battery is working, it involved the heat generation. As a result, the separator formed by the stretchable polyethylene resin or so is also heated. The separator formed by the stretchable polyethylene resin generally easily shrink even at the temperature below 150° C., hence it easily causes short circuit of the battery. Also, when the battery is penetrated by a projection having the sharp shape such as nails or so (for example, during the nail penetration test), the short circuit takes place immediately, and generates the reaction heat thus the short circuit area enlarges.
Therefore, in order to solve such problems, it is proposed to provide the porous film as the protective film on the electrode surface. By providing the porous film, the fall off of the active material during the formation of the battery, and the short circuit while the battery is working are prevented. Furthermore, since it is a porous film, the electrolyte solution permeate into the protective film hence it does not interfere the battery reaction.
For example, the patent document 1 discloses a porous film formed by using polyvinylidene fluoride as the binder and the fine particle slurry including the fine particle such as alumina and silica or so. The patent document 2 discloses a porous film formed by using a heat crosslinking resin such as polyacrylonitrile or so as the binder. Also, the patent document 3 discloses a porous film formed by using polystyrene, polyethylene, or poly acrylic acid or so.
However, when the polymer described in the above patent documents are used as the binder, during the step of winding the electrode coated with the porous film by the winding roll, the washing step of the roll is needed since the porous film adheres to the winding roll. Also, the ability as the protective film tends to deteriorate since the porous film detaches while winding.